We were replacing the valve cover gaskets on my dad's '72 C10 this weekend, and my dad told me to check the head castings, cause he wants to know how many cc they are, the last 3 digits are 820, I checked mortec but came up with nothing for that #, there is a 920 casting that is for '71-'72 350's ( no other info ither ) but his heads are deffinetly 820's, any other info? Thanks in advance.